1. Field of the Invention
The present invention relates to a process for producing a dicarboxylic acid. More specifically, it relates to a process for producing a dicarboxylic acid by subjecting a cycloalkane to an oxidative cleavage reaction with oxygen in the presence of a catalyst using a continuous reactor to thereby yield a corresponding dicarboxylic acid. Such dicarboxylic acids are useful as, for example, raw materials for polyamides and polyesters, additives for polymers, and intermediate materials for fine chemicals. Among them, adipic acid is typically important as a raw material for nylon 66 (polyamide 66).
2. Description of the Related Art
Certain processes of oxidatively cleaving a mixture of a cycloalkanone and a cycloalkanol are known as processes for producing dicarboxylic acids. For example, adipic acid, a raw material for polyamides, is produced by a process of converting cyclohexane by oxidation with air into a mixture of cyclohexanone and cyclohexanol, and oxidizing the mixture with nitric acid. However, this process invites large amounts of nitrogen oxides, which are believed to be global warming gases, during oxidation with nitric acid and requires enormous facilities and efforts for disposal of the nitrogen oxides.
As a possible solution to the problem, a process of directly oxidizing a cycloalkane with oxygen to thereby yield a corresponding dicarboxylic acid has been investigated as a process without by-production of nitrogen oxides. The process, if feasible, can markedly reduce production process steps and production cost of dicarboxylic acids.
For example, processes of oxidizing cyclohexane in one step to yield adipic acid have been studied since 1960s (e.g., Japanese Unexamined Patent Application Publication No. 49-100022, PCT International Publication No. WO 9407834, Japanese Patents No. 3197518 and No. 3056790). However, no plant has been launched in commercial production. The reasons are as follows. These processes have been conventionally studied mainly using batch-system reactors in which if the conversion from cyclohexane increases for increasing productivity of dicarboxylic acid, a reaction time increases, and the increased reaction time invites increased by-production of glutaric acid, succinic acid, and other dicarboxylic acids than the target adipic acid and increased by-production of esters, lactones, high-boiling compounds, and other by-products. Accordingly, the processes require complicated purification process steps, invite a decreased utilization of cyclohexane to thereby invite increased production cost of adipic acid. In addition, such by-products deteriorate the catalytic activity. Further, the batch-system reactors require excessively large cost of construction and equipment of plants and are low in operability in commercial production.